Anyway I have the B's and love em...Tightens thing up nicely...
Rode in cars with both and the C's are a bit firmer (harsher). Firmer than I wanted anyway esp. for city driving or rough roads...
PART NUMBER MODEL YEAR AMOUNT LOWERED (V-8) SPRING RATES (LB./IN.)
FRONT REAR FRONT REAR
M-5300-B (6) 1979-02 (5) 7/8" (1) 1/2" (2) 425/530 200/300
M-5300-C (6) 1979-02 (5) 7/8" (1) 1/2" (2) 650 200/300
You have variable spring rates in the B series, meaning a more comfortable ride under different driving conditions. The 650 front spring rate on the C series provides for a harsher ride but crisper, tighter handling.
I think the price is the same, so whatever you are going for (some handling and a nice ride OR great handling and an okay ride), the price isn't bad (when compared to Eibach, BBK, etc).
I had the C springs on my 94 GT and LOVED them, now I have a 91 GT and I am going to put them on that, I am a return customer of the C spring and I really like them alot, they tend to retain the factory ride feel on the 94 I had, but they cornered a TON better!!!
If you are looking into new springs - do some homework on H&R. IMO - they are a better value than the Ford and Eibach stuff (having both before). And when you are swapping springs, UPGRADE THE SHOCKS/STRUTS! That is a must! Another part many people forget to put on is a bump-steer kit (adj. tie-rod ends). That makes a big difference in a lowered car. Not only did I notice less harshness, but less under-steer too.